1. Field of the Invention
The present invention relates to an optical amplifier and, in particular, it relates to a multistage optical amplifier that amplifies a signal having a plurality of optical wavelengths (hereinafter referred to as a “main signal” or a “channel”), and the like, in the wide bandwidth used in a WDM (wavelength Division Multiplexing) system.
2. Description of the Related Art
In recent years, as the Internet has rapidly become widespread, an optical wavelength division multiplexing transmission technique that enables large-capacity and long-distance transmission has attracted attention. In the optical amplification used for the long-distance WDM transmission system, an erbium doped fiber amplifier (EDFA), in which erbium is doped into optical fiber cores, is used. Further, as the number of wavelengths is increased due to the increase of speed and capacity of communications, a Raman fiber amplifier that has a wider amplification bandwidth than the EDFA is also used.
As the wavelength bandwidth to be used has expanded, a difference (tilt) of the level between the shortest wavelength channel and the longest wavelength channel of the WDM signal has tended to increase. The tilt is generated as a result of fiber loss wavelength characteristics of transmission paths, characteristics of the amplification factor of an optical amplifier, a nonlinear optical effect due to a stimulated Raman effect and the like and is accumulated due to the increase of transmission distances, the number of spans and the like.
In an optical transmission system, in order to maintain its transmission quality, it is necessary to ensure that the optical signal-to-noise ratio (OSNR) is higher than a predetermined value. However, as the OSNR of the channels, the level of which is reduced due to the tilt described above, is reduced significantly, the transmission distance, the number of transmission spans or the number of multiplexed wavelengths (or, in other words, the transmission bandwidth) will be restricted.
Therefore, conventionally, in order to compensate for the tilt that is generated and accumulated in the transmission path or the optical amplifier, every time an optical wavelength multiplexed signal is transmitted along several spans, the signal is once demultiplexed into separate channel signals by an optical demultiplexer (optical DEMUX), then, the power level of each channel signal is adjusted by an optical variable attenuator (optical ATT) so that the power level is equalized between the channels and, then, the channel signals are converted into a WDM signal again by an optical multiplexer (optical MUX).
In another conventional technique, in order to compensate for the tilt of a WDM signal collectively in a one-stage optical amplifier, the tilt generated in the amplifier is detected by using predetermined false signals and, then, based on the detected value of the tilt, a gain is controlled by adjusting pumping light power and an amount of attenuation is controlled by an optical variable attenuator in a combined manner so that the optical amplifier achieves a flat gain (see Documents 1 and 2).
Document 1:                Japanese Unexamined Patent Publication 2003-51791        
Document 2:                Japanese Unexamined Patent Publication 2000-91683        
However, in the device configuration using optical demultiplexers and optical multiplexers every several spans for level adjustment of separate channels as in the conventional example, there is a problem in that the size and cost of the device is increased. In this case, as optical amplifiers are typically required to compensate for insertion losses generated when the channel signals pass through the optical demultiplexers and optical multiplexers, there is also a problem in that amplified spontaneous emission light (ASE light) is generated in these optical amplifiers and superimposed on the main signal to degrade the OSNR and, thus, the transmission quality is degrated.
Further, when the pumping light power is controlled based on the detected value of the tilt in the one-stage optical amplifier, there is a problem in that flat gain characteristics can be achieved at a particular amplification gain value corresponding to a specific pumping light power and the maximum gain of the optical amplifier is limited, in a pinpoint manner, to such a particular amplification gain value.
In this case, as a reference operating point is set to a level lower than said maximum gain, by some dB, in consideration of fluctuations of the input signal level, the gain of the optical amplification is further restricted. As a result, when this optical amplifier is used as a first-stage amplifier in a multistage configuration, there is a problem in that the OSNR is not improved much and, therefore, good transmission quality cannot be provided.